In a magnetic disk unit, a plurality of circular magnetic disks are fixed to the drive shaft of the spindle motor and rotated and driven together with the spindle motor. An air layer (air bearing) is generated between the surface of a rotating magnetic disk and the slider with a magnetic head mounted and the magnetic head relatively moves on the magnetic disk with a fine interval held. The surface of the magnetic disk is finely textured so as to prevent it from sticking to the slider and improve magnetic characteristics (characteristics of flying height).
When the magnetic head relatively moves on the magnetic disk, an offtrack is generated due to the fluttering phenomenon caused by vibration of the bearing in the spindle motor and a minute bending of the magnetic disk, a texture mark on the magnetic disk, and vibration of servo information which is recorded on the magnetic disk beforehand.
Therefore, the track follow-up control is executed so as to demodulate a track position signal from the reproduced servo information during data recording and reproducing and suppress the amount of offtrack on the basis of the signal. In the embedded servo system, the servo information area is arranged at each fixed interval on the same recording track as the data recording area and the pattern for detection of servo information is recorded in the area beforehand.